1. Field of the Invention
The present invention generally relates to a driving circuit with multistage charging for liquid crystal displays, and more particularly to a driving circuit, which is able to perform charge-sharing and pre-charge on the pixels of liquid crystal displays.
2. Description of the Prior Art
Recently, due to the fast development in electro-optics and semiconductor technology, the related techniques compatible with multimedia applications of displays have grown rapidly. The advantages of a liquid crystal display (to be abbreviated as LCD here below), in contrast to conventional displays, include smaller size and better moving picture quality and image display characteristics. At present, the technical fields according to LCDs, especially those related to the driving circuit of a thin film transistor-liquid crystal display (TFT-LCD), have become the key techniques.
The conventional techniques for driving TFT-LCDs, wherein the voltage output range of a data driver varies with the polarity alternating of pictures alternating, suffer from high power dissipation and include: (1) charge-sharing by using a large external storage capacitor; and (2) pixels of different polarities driven by different data drivers. However, there still exist problems due to gray-scale voltage imprecision and latch up at the output.
As mentioned above, power dissipation and gray-scale voltage precision are two important references by which the performance of a driving circuit of a LCD is judged. It is well known that the output power dissipation is proportional to the square of voltage swing. Please refer to FIG. 1A and FIG. 1B, which respectively illustrate a schematic driving circuit of a liquid crystal display using vertical signals and the relation between the liquid crystal applied voltage and liquid crystal transmittance in the prior art. As shown in FIG. 1A and FIG. 1B, the driving circuit must follow the polarity alternating of data signals in order to change its range of dynamic operation. Even though only the positive half cycle or the negative half cycle of the voltage range is used, as shown in FIG. 2, the voltage range of dynamic operation at the output stage of the entire driving circuit must be two times the voltage range as the uni-polarity case is concerned. In other words, the output must be able to provide considerably large voltage swing and thus the power dissipation is increased.
One of the approaches to reduce the power dissipation is to lower the voltage swing at the output stage. To achieve such object, several improved driving methods, as shown in FIG. 3A, FIG. 3B and FIG. 4, have been claimed. Among them, as can be seen in FIG. 3A, a considerably large external storage capacitor 66 may be coupled between ground and common node 65, and the capacitance of the external storage capacitor 66 is much larger than the sum of all the capacitance of the pixels. The external storage capacitor 66 averages the voltages, over time, applied to the columns of the array. Before the data are written (as presented by intervals t0xcx9ct1 and t2xcx9ct3 in the waveform timing diagram shown in FIG. 3B), SELECT goes high, shorting column 2 through mutliplexer 78 to external storage capacitor 66 and resulting in charge-sharing. In practice, the value of external capacitor 66 is large enough to sink such charges without producing a noticeable variation in the voltage there across, therefore the voltage on Column 2 drops to approximately ground potential, as shown in FIG. 3B. In such manner, the voltage swing is reduced.
Please further refer to FIG. 4, which is a circuit diagram in accordance with U.S. Pat. No. 5,748,165, wherein the positive-polarity voltage and negative-polarity voltage are supplied by different output terminals, respectively. For example, the positive-polarity voltage is supplied by the upper output terminal (OP+) and the negative-polarity voltage is supplied by the lower output terminal (OPxe2x88x92). In this manner, the output voltage swings at both the upper output and the lower output are decreased to half as compared to the conventional technique, as shown in FIG. 1.
However, the above improved driving methods remain unable to solve two major problems such as:
(1) Gray-scale voltage imprecision: Within the time constant xcfx84=R*C, the response of an STC circuit to an input signal can never reaches the initial value of the input signal. The difference between the initial value and the response value decreases with time, and thus the voltage deviation the liquid crystal cell holds decreases. However, in the methods as illustrated in FIG. 1A and FIG. 3A, the initial voltage and response voltage have opposite polarities. Therefore, the voltage deviation the liquid crystal cell holds can not be neglected.
(2) Latch up at the output: As can be seen in FIG. 4, when the liquid crystal cell is turned on, the upper output (OP+) xe2x80x9cseesxe2x80x9d the negative-polarity voltage of the prior frame, resulting in latch up.
Accordingly, in order to overcome the above problems, the present invention provides an improved circuit, which is able to perform charge-sharing and pre-charge at the same time.
It is a main object of the present invention to provide, a multistage charging driving circuit for liquid crystal displays, characterized in that the pixel is charged to a fixed value by performing charge-sharing and pre-charge before the next data are written. Since the charged pixel and the next data have the same polarity, the latch up at the output of the driving circuit can be prevented. In addition, the fixed voltage value is set to be around the median gray-scale, therefore, during the same data-write time, the median gray-scale voltage has better accuracy than in the prior arts.
It is another object of the present invention to provide a multistage charging driving circuit for liquid crystal displays, characterized in having better accuracy in median gray-scale voltage and lower power dissipation, preventing the latch up at the output of the driving circuit.
In order to achieve the foregoing objects, the present invention provides an improved circuit, which is able to perform charge-sharing and pre-charge on the pixels of liquid crystal displays, comprising (1) switches for performing charge-sharing and pre-charge; and (2) voltage levels for pre-charge (M(+) and M(xe2x88x92), the output of M being positive or negative-polarity voltage) and voltage selection controllers (C1xcx9cCM). All these components can be implemented outside the pixel region.
It is preferable that, according to the present invention, the circuit being able to perform charge-sharing and pre-charge can be further coupled to an external storage capacitor with large capacitance in order to reduce the voltage swing at the output.